Porous iron article and method of making same



Jan. 16, 1940. V v LENEL 2,187,589

POROUS IRON ARTICLE AND METHOD OF MAKING SAME Filed Nov. 3, 1958 2Sheeis-Sheet 1 o .0002 .0004- .0006.0008 .00l0 .00l2 .00l4- .00l6 .0010.0020 .002?- .0000 .001. .0026 .0050 .0057.

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(mm m Winemm G'RAMQ AHVENTOR Fm'rz. V.v LENEL Patented Jan. 16, 1940PATENT OFFICE POROUS IRON ARTICLE AND METHOD OF MAKING SAME Fritz V.Lenel, Dayton, Ohio, asslgnor to General Motors Corporation, Detroit,Mich, a corporation of Delaware Application November 3, 1938, Serial No.238,527

12 Claims.

This invention relates to porous sintered iron articles and moreparticularly to a method of treating such articles to make the sameresistant toward atmospheric corrosion.

An object of the invention is to provide a method for treating articlesfabricated from powdered iron whereby the entire porous structure of thearticle is coated over the entire surface there- 'of with a corrosionresistant film or layer.

In carrying out the above object it is a furth r object of the inventionto subject the porous iron article to treatment in an atmosphere of achemical compound or element which reacts chemically with the surface ofthe article to form a corrosion resistant film or coating thereover.

It is still a further object in some instances to treat the porous ironarticle with steam, at a temperature above 1000" F., for accomplishingoxidation of the metal surface thereof.

A further object is to provide a new article of manufacture whichconsists of an object formed of porous sintered iron in which the entireporous surface thereof is coated with a corrosion resistant filmresulting from a chemical reaction with the iron. 7

Another object of the invention is to provide a method whereby a porousiron article may be made corrosion resistant and simultaneouslyincreased in hardness as well as bettering other physical propertiesthereof.

Further objects and advantages of the present invention will be apparentfrom the following description, reference being had to the accompanyingdrawings wherein a preferred embodiment of the present invention isclearly shown.

In the drawings:

Fig. 1 is a graph'showing the change in diameter of a porous ironbriquette due to steam treatment thereof.

Fig. 2 is a graph showing the change in weight of a porous ironbriquette due to steam treatment thereof.

Fig. 3 is a graph showing stress strain diagrams for treated anduntreated iron, and

Fig. 4 is a schematic layout of the apparatus used for steam treatingporous iron,

Porous iron articles formed by briquetting and sintering powdered iron,such as sponge iron, comminuted iron, electrolytic iron, etc., have manyuses in industry. These porous sintered articles may be utilized assubstitutes for articles formed by conventional methods from cast ironor steels, etc., and in each instance can be fabricated in substantiallythe exact shape and size without subsequent machining. The

porous iron articles have the further advantage of being capable ofabsorbing a lubricant and thereby providing a self lubricating surfacewhen the same is required.

One of the past difliculties in the use of porous iron has been therapid corrosion of the iron due to exposure to atmospheric air. Theporous spongy structure of the iron, due to its large area, acts verysimilarly to an activated surface, and corrosion is a great deal moremarked than in cases where solid bodies of iron are used. This rapidcorrosion has presented difliculties which heretofore have not beenovercome, and as a result have impeded the widespread use of porousiron. The present invention provides a method for making porous ironarticles which are substantially unafiected by atmospheric gases andwhich, due to the treatment thereof, have more desirable physicalproperties than untreated porous iron articles.

I am aware of the Bower-Barf! process in which iron articles aresubjected to steam to form an outside coating thereover. I am also fullycognizant of many other processes for effecting corrosion resistingfilms on iron and steel. In each of these cases, however, the only partof the ferrous article to be coated is the surface thereof which due toits non-porous nature requires long periods of treatment. In myinvention I propose to subject porous iron articles to a steam treatmentfor a relatively short period of time, and ranging in temperature from1000 F. upwards to 1500 F. This treatment differs from the. conventionaltreatments inasmuch as the entire surface of the porous iron article iscoated or filmed with the oxide. Thus the wall of each intersticial poreis oxidized to prevent further atmospheric oxidation thereof. Preferablymy process is carried out at a temperature of approximately 1050 F. fora period of twenty minutes, although these figures are not in any waylimiting as it is apparent that the length of treatment and temperaturethereof may be varied within wide limits and still obtain satisfactoresultsf One method of steam treatment is shown in Fig. 4 wherein steamenters pipe 20 and passes through a preheating or superheating coil 22disposed within a closed furnace 24 to bring the steam temperature up tothe temperature of the furnace. The articles 26 to be treated are placedon a rack 28 and maintained at a preferred temperature of 1050" F. Anoutlet 30 is provided for. the steam and sufflcient pressure is used tocause a slow circulation of the vapor through the furnace. z

I have further found that such treatment has very beneficial effects oncertain physical proptreatment.

It has further been found that the porosity of a porous iron piece isdecreased by steam treatment. This may be explained by the fact that theoxide film decreases the size of the pores of the piece. Anotherphysical property which is materially improved is the yield strength incompression of the piece. In Fig. 3, thestress strain diagrams oftreated and untreated porous iron are shown together with the yieldpoints as determined by the ofiset' method (oifset of gauge length). Itwill be observed that the yield point \is materially increased by steamtreatment. In

this instance the iron article was fabricated from a mixtureof 98 partssponge iron, 2 parts graphite and 1 parts zinc stearate or stearic acid,the function of the fatty acid or salt thereof being to lubricatethebriquetting die. As-a result of the oxidation there is a growth of thearticle during steam treatment which causes the article to change insize in every direction. This growth necessitates briquetting an articleof the desired shape but less in size in all directions than the,article ultimately desired, and then by selective steam treatmentcausing the article to grow to the size ultimately desired. This growthhas been rather accurately I determined, and a graph showing arepresentative curve is-shown in Fig. 1.

In some instances where extreme accuracy is desired the iron article maybe subjected to a machine operation, such as grinding or turning, etc.,to bring the same to the exact size. In this case the steam treatmentmaybe carried out in either one of two alternative methods. First thearticle may be briquetted and sintered to the desired size and thensteam treated for a suitable period to cause an increase in sizethereof. This increased size may then be ground ofi until the article isthe exact size, after which a short steam treatment may be used tooxidize 'the external surface where the oxide. has beenground off. Inthis treatment the steam oxidize the intersticial pores in thepreliminary oxidation step, whereby the final oxidation step does notcause any substantial additional growth at the interior of the article.The second method of obtaining the exact size,when the piece is to bemachined, is to perform the grinding operation prior to the steamtreatment and grind the surface of'the article sufflciently so that byselective steam treatment the growth thereof will bring the article backto the exact size desired The 'amount of oxidation, or stateddifferently, the extent of oxidation may be. measured ac-' curately bythe change in weight of the article, such change in weight per unit timebeing shown on the chart in Fig. 2 for a given temperature. In figuringout the percentage of oxide present, magnetic iron oxide (F8304) shouldbe considered as a base. Magnetic iron oxide is much more adherent thanthe usual red iron oxide and forms a much improved corrosion resistantfilm.

Instead of utilizing steam, heated air, carsired, also the time andtemperature should be varied in accordance with the oxidizing quality ofthe gaseous medium. Thus when using air or oxygen the temperature isbest maintained toward the lower limits of temperature range, namely,1000 to 1500 F. When carbon dioxide is used, however, the temperatureshould approach the upper limits since carbon dioxide is a sloweroxidizer-than air or oxygen containing vapors or gases.

-'From the foregoing it is apparent that the treatment described hereinfor porous iron articles provides a protective coating thereover againstcorrosion and simultaneously improves certain physical characteristicsof the porous iron. Similar treatment can be accorded various porousiron alloy articles which include graphite and metallic alloyingingredients, copper, nickel, manganese, etc., the description beinglimited to iron v merely for illustrative purposes.

While the embodiment of the present invention ing, treating a porousiron article with steam at a temperature of 1050 F. and for a period ofapproximately 20 minutes, whereby the surface only of the iron isoxidized for producing a corrosion resistant oxide iilm thereover.

2. In a method of fabricating articles from powdered iron, the stepscomprising, briquetting powdered iron under high pressure to the shapedesired but of a slightly smaller size in all directions than the sizeultimately desired, sintering the briquettetc form a strong porousarticle,

and then subsequently treating the porous iron article with steam at atemperature between 1000 F. and 1500 F. and for a time sufiicient tooxidize the surface only of the iron'and to cause the article'toincrease in size to the size ultimately desired and simultaneouslyprovide a corrosion resistantsurface thereover.

3. In a method of fabricating articles from powdered iron, the stepscomprising, briquetting powdered iron under high pressure, sintering thebriquette to form a strong porous article, machining the article thusformed to a size less in all directions than the size ultimatelydesired, and then treating the machined article with steam at atemperature in excess of 1000 F. and for a time sumcient to oxidize thesurface only of the iron and cause the article to increase in -size tothe size ultimately desired, and simultaneously provide a corrosionresistant surface thereover.

4. In a method of fabricating articles from powdered iron, the stepscomprising, briquetting powdered iron under high pressure to the shapedesired, sintering the briquette formed into a strong porous article,treating the article with steam at a high temperature and for asuiiicient time to. oxidize the surface only of the iron thereby causingthe article to increase in size, machining the article to the desiredsize, and then treating the article in high temperature steam for such ashort time as to only oxidize the external surface thereof for providinga corrosion resistant film thereover.

5. As a new article of manufacture, an object of porous sintered ironhaving high strength and having substantially the entire porous surfaceonly thereof coated with a corrosion resistant oxide film 0f superficialdepth formed by the chemical reaction between iron and steam.

6. As a new article of manufacture, an object of porous sintered ironhaving high strength and having substantially the entire porous surfaceonly thereof coated with F8304 in a film of superficial depth.

'7. In a method of preventing corrosion of porous sintered ironarticles, that step comprising, treating a porous iron article in anatmosphere of an oxidizing gas at a temperature in a neighborhood of1,050 F. and for a period of about 20 minutes to cause substantially theentire porous surface only of the iron to be oxidized to a superficialdepth.

8. In a method of preventing corrosion of porous sintered iron articles,that step comprising, treating a porous iron article in an atmosphere ofair at a temperature in the neighborhood of 1.050 F. and for a period ofabout 20 minutes 35 for causing substantially the entire porous sur- 9.In a method for treating porous ferrous articles, that step comprising,subjecting a porous iron article to a steam treatment at a temperatureof about 1,050 F. and for a period of about 20 minutes for oxidizing theentire porous surface only thereof to a superficial depth andsimultaneously to improve certain physical characteristics thereof.

10. In a' method of fabricating articles from powdered iron tosubstantially the exact size desired, comprising the steps ofbriquetting powdered iron under high pressure to the shape desired butof a slightly smaller size in all directions than the size ultimatelydesired, sintering the briquette under non-oxidizing conditions into astrong porous iron article, and then treating the article with anoxidizing gas at a high temperature and for such a time as to oxidizethe external surface only thereof for causing the article to increase insize to the size ultimately desired and simultaneously provide a porouscorrosion resistant surface thereover.

11. As a new article of manufacture, an object of porous sintered ironhaving high strength and having substantially the entire porous surfacethereof coated with a corrosion resistant oxide film of superficialdepth formed by the chemical reaction between iron and air.

12. As anew article of manufacture, an object of porous sintered ironhaving high strength and having substantially the entire porous surfaceonly thereof coated with a black oxide of iron in a film of superficialdepth.

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